Complex systems is a concept that belongs to the field of complexity theory, which is founded on the assumption that reality cannot be understood properly only terms of cause and effect, in the manner of the classical analytical method.

Complex systems can be simply defined as any system containing a large number of elements like language, economics or biology. These systems cannot be properly understood from the perspective of Newtonian physics because complex systems interact with the environment and therefore tend to instability. When the number of elements forming a system is smaller, it is more predictable and easy to describe in conventional terms.

However, it is very important to bear in mind that, as scientist Paul Cilliers claims in his book Complexity and Postmodernism, a large number of elements is not enough to constitute a complex system, the elements have to interact with the environment as i mentioned before. For instance, there are many grains of sands in a beach but this does not make them a complex systems because they do not interact dynamically with the environment. Hence, complex systems are not necessarily more diffcult to understand than non-complex systems.